The present invention relates to a second-order static magnetic field correcting method and an MRI (magnetic resonance imaging) apparatus, and more particularly to a second-order static magnetic field correcting method for correcting second-order static magnetic field components to improve homogeneity in an MRI apparatus and an MRI apparatus that can implement the method.
The static magnetic field of an MRI apparatus should be homogeneous. Homogeneity of the static magnetic field is achieved by mechanical shimming or by adding small pieces of magnet, iron or the like.
A metal mass (e.g., an automobile) moving near the MRI apparatus or an environment change (e.g., a change in temperature) varies the static magnetic field, and second-order static magnetic field components are generated.
Pulse sequences that observe gradient echoes, such as one according to GRASS (gradient recalled acquisition in the steady state) or SPGR (spoiled GRASS), are very sensitive to the static magnetic field components, and the generation of the second-order static magnetic field components leads to degradation of image quality.
It is therefore an object of the present invention to provide a second-order static magnetic field correcting method for correcting second-order static magnetic field components to improve homogeneity and an MRI apparatus that can implement the method.
In accordance with its first aspect, the present invention provides a second-order static magnetic field correcting method characterized in: disposing a first circular loop coil and a second circular loop coil at positions spaced in a static magnetic field direction to be symmetrical with respect to a center of an imaging region of an MRI apparatus; disposing a third circular loop coil and a fourth circular loop coil having a larger diameter than that of said first and second circular loop coils at positions spaced in the static magnetic field direction to be symmetrical with respect to the center of said imaging region; generating a first corrective magnetic field and a second corrective magnetic field in the same direction by said first and second circular loop coils; and generating a third corrective magnetic field and a fourth corrective magnetic-field in the same direction and opposite to said first corrective magnetic field by said third and fourth circular loop coils; thereby correcting second-order static magnetic field components in the static magnetic field direction.
In this specification, by a xe2x80x9csecond orderxe2x80x9d is meant a quadric function of a position in a static magnetic field direction. By a xe2x80x9czeroth orderxe2x80x9d is meant independence of the position in the static magnetic field direction. Furthermore, by a xe2x80x9cfirst orderxe2x80x9d is meant a linear function of the static magnetic field direction.
In the second-order static magnetic field correcting method of the first aspect, since the direction of the first and second corrective magnetic fields generated by the first and second circular loop coils is opposite to the direction of the third and fourth corrective magnetic fields generated by the third and fourth circular loop coils, zeroth-order corrective magnetic field components in the static magnetic field direction can cancel one another, and thus the zeroth-order static magnetic field components are not affected. On the other hand, since the zeroth-order corrective magnetic field components are independent of the second-order corrective magnetic field components, the second-order corrective magnetic field components remain even after the zeroth-order corrective magnetic field components have canceled one another. Therefore, the second-order corrective magnetic field components can cancel the second-order, static magnetic field components, and homogeneity of the static magnetic field can be improved.
Since the first and second corrective magnetic fields are in the same direction, these fields will not generate first corrective magnetic field components. Similarly, since the third and fourth corrective magnetic fields are in the same direction, these fields will not generate first corrective magnetic field components.
In accordance with its second aspect, the present invention provides the second-order static magnetic field correcting method of the aforementioned configuration, characterized in that said first and second circular loop coils are disposed substantially coplanar with gradient coils for the static magnetic field direction and outside said gradient coils.
In the second-order static magnetic field correcting method of the second aspect, since the first and second circular loop coils are disposed substantially coplanar with gradient coils for the static magnetic field direction and outside the gradient coils, symmetry with respect to the gradient coils for the static magnetic field direction is conserved; moreover, the first and second corrective magnetic fields are in the same direction, and therefore the gradient magnetic fields are not substantially affected. Furthermore, since circular loop coils, which have no return path, exhibit better linearity than that of gradient coils having return paths, linearity of the gradient magnetic fields is not substantially affected.
In accordance with its third aspect, the present invention provides the second-order static magnetic field correcting method of the aforementioned configuration, characterized in that said third and fourth circular loop coils are disposed surrounding magnetism conditioning plates.
In the second-order static magnetic field correcting method of the third aspect, since the third and fourth circular loop coils are disposed surrounding magnetism conditioning plates, the space for installing the circular loop coils can be easily secured. Moreover, there is no need for concern about coupling with the gradient coils.
If the third and fourth circular loop coils are disposed so that the conditions of Helmholtz coils are fulfilled, second-order corrective magnetic field components generated by the third and fourth circular loop coils can be ignored. Therefore, in order to cancel second-order static magnetic field components, only the second-order corrective magnetic field components generated by the first and second circular loop coils need to be adjusted, and thus the adjustment can be done easily.
In accordance with its fourth aspect, the present invention provides the second-order static magnetic field correcting method of the aforementioned configuration, characterized in that at least one of the ratio of electric currents and the turns ratio of said first through fourth circular loop coils is determined so that zeroth-order corrective magnetic field components in the static magnetic field direction cancel one another.
In the second-order static magnetic field correcting method of the fourth aspect, since the ratio of electric currents or the turns ratio is adjusted so that the zeroth-order corrective magnetic field components are canceled out, only the electric current values need to be adjusted to cancel the second-order static magnetic field components while maintaining the ratio of corrective electric currents, and thus the adjustment can be done easily.
In accordance with its fifth aspect, the present invention provides a second-order static magnetic field correcting method, characterized in: disposing three RF probes at different positions in a static magnetic field direction of an MRI apparatus, each of which probes has a small phantom capable of emitting an FID (free induction decay) signal and a small coil combined; transmitting RF pulses from said RF probes and receiving FID signals at a time when a magnetic field variation is to be measured; determining frequencies f1, f2 and f3 from the FID signals; determining a second-order static magnetic field component, xcex22 by solving the following simultaneous equations:
f1=xcex20+xcex21xc2x7r1+xcex22xc2x7r12
f2=xcex20+xcex21xc2x7r2+xcex22xc2x7r22,
f3=xcex20+xcex21xc2x7r3+xcex22xc2x7r32
wherein the positions of said RF probes are designated by r1, r2 and r3; and adjusting corrective magnetic fields based on said second-order static magnetic field component xcex22.
In the second-order static magnetic field correcting method of the fifth aspect, since corrective magnetic fields are adjusted by disposing RF probes to measure the second-order magnetic field components at an appropriate time, the correction can be suitably achieved.
In accordance with its sixth aspect, the present invention provides a second-order static magnetic field correcting method, characterized in: disposing three RF probes at different positions in a static magnetic field direction of an MRI apparatus, each of which probes has a small phantom capable of emitting an FID signal and a small coil combined; transmitting RF pulses from said RF probes and receiving FID signals at a time when a reference magnetic field is to be measured; determining reference frequencies f1r, f2r and f3r from the FID signals; transmitting RF pulses from said RF probes and receiving FID signals at a time when a magnetic field variation is to be measured; determining frequencies f1, f2 and f3 from the FID signals; determining a second-order magnetic field variation xcex12 by solving the following simultaneous equations:
f1xe2x88x92f1r=xcex10+xcex11xc2x7r1+xcex12xc2x7r12
f2xe2x88x92f2r=xcex10+xcex11xc2x7r2+xcex12xc2x7r22
f3xe2x88x92f3r=xcex10+xcex11xc2x7r3+xcex12xc2x7r32
wherein the positions of said RF probes are designated by r1, r2 and r3; and adjusting corrective magnetic fields based on said second-order magnetic field variation xcex12.
In the second-order static magnetic field correcting method of the sixth aspect, since corrective magnetic fields are adjusted by disposing RF probes to measure a reference static magnetic field as a reference frequency at an appropriate time, disposing the RF probes to measure the static magnetic field as a frequency at a later appropriate time, and measuring a second-order magnetic field variation from their difference, the correction can be suitably achieved.
In accordance with its seventh aspect, the present invention provides the second-order static magnetic field correcting method of the aforementioned configuration, characterized in: determining a zeroth-order magnetic field variation xcex10, and adjusting a transmit frequency for an RF pulse and a receive detection frequency for an NMR signal based on said zeroth-order magnetic field variation xcex10.
In the second-order static magnetic field correcting method of the seventh aspect, a zeroth-order static magnetic field variation can be compensated by correction of the transmit frequency for an RF pulse and the receive detection frequency for an NMR signal.
In accordance with its eighth aspect, the present invention provides the second-order static magnetic field correcting method of the aforementioned configuration, characterized in: determining a first-order magnetic field variation xcex11, and adjusting a gradient electric current based on said first-order magnetic field variation xcex11.
In the second-order static magnetic field correcting method of the eighth aspect, a first-order static magnetic field variation can be compensated by correction of the gradient electric current.
In accordance with its ninth aspect, the present invention provides the second-order static magnetic field correcting method of the aforementioned configuration, characterized in that said MRI apparatus is an open-type MRI apparatus that generates the static magnetic field in a vertical direction.
In the second-order static magnetic field correcting method of the ninth aspect, homogeneity of the static magnetic field in an open-type MRI apparatus in which homogeneity of the magnetic field is obtained by mechanical shimming or by adding small pieces of magnet, iron or the like can be improved.
In accordance with its tenth aspect, the present invention provides an MRI apparatus characterized in comprising: a first circular loop coil and a second circular loop coil disposed at positions spaced in a static magnetic field direction to be symmetrical with respect to a center of an imaging region, for generating a first corrective magnetic field and a second corrective magnetic field in the same direction; a third circular loop coil and a fourth circular loop coil having a larger diameter than that of said first and second circular loop coils, disposed at positions spaced in the static magnetic field direction to be symmetrical with respect to the center of said imaging region, for generating a third corrective magnetic field and a fourth corrective magnetic field in the same direction and opposite to said first corrective magnetic field; and circular loop coil driving means for applying corrective electric currents to said first through fourth circular loop coils to generate said first through fourth corrective magnetic fields.
In the MRI apparatus of the tenth aspect, the second-order static magnetic field correcting method as described regarding the first aspect can be suitably implemented.
In accordance with its eleventh aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in that said first and second circular loop coils are disposed substantially coplanar with gradient coils for the static magnetic field direction and outside said gradient coils.
In the MRI apparatus of the eleventh aspect, the second-order static magnetic field correcting method as described regarding the second aspect can be suitably implemented.
In accordance with its twelfth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in that said third and fourth circular loop coils are disposed surrounding magnetism conditioning plates.
In the MRI apparatus of the twelfth aspect, the second-order static magnetic field correcting method as described regarding the third aspect can be suitably implemented.
In accordance with its thirteenth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in that at least one of the ratio of electric currents and the turns ratio of said first through fourth circular loop coils is determined so that zeroth-order corrective magnetic field components in the static magnetic field direction cancel one another.
In the MRI apparatus of the thirteenth aspect, the second-order static magnetic field correcting method as described regarding the fourth aspect can be suitably implemented.
In accordance with its fourteenth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in comprising: frequency acquiring means for, under the condition that three RF probes are disposed at different positions in the static magnetic field direction, each of which probes has a small phantom capable of emitting an FID signal and a small coil combined, transmitting RF pulses from said RF probes and receiving FID signals, and determining frequencies f1, f2 and f3 from the FID signals; and corrective magnetic field adjusting means for determining a second-order static magnetic field component xcex22 by solving the following simultaneous equations:
xe2x80x83f1=xcex20+xcex21xc2x7r1+xcex22xc2x7r12
f2=xcex20+xcex21xc2x7r2+xcex22xc2x7r22
f3=xcex20+xcex21xc2x7r3+xcex22xc2x7r32
wherein the positions of said RF probes are designated by r1, r2 and r3, and adjusting the corrective magnetic fields based on said second-order static magnetic field component xcex22.
In the MRI apparatus of the fourteenth aspect, the second-order static magnetic field correcting method as described regarding the fifth aspect can be suitably implemented.
In accordance with its fifteenth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in comprising: reference frequency acquiring means for, under the condition that three RF probes are disposed at different positions in the static magnetic field direction, each of which probes has a small phantom capable of emitting an FID signal and a small coil combined, transmitting RF pulses from said RF probes and receiving FID signals at a time when a reference magnetic field is to be measured, and determining reference frequencies f1r, f2r and f3r from the FID signals; frequency acquiring means for, under the condition that three RF probes are disposed at different positions in the static magnetic field direction, each of which probes has a small phantom capable of emitting an FID signal and a small coil combined, transmitting RF pulses from said RF probes and receiving FID signals at a time when a magnetic field variation is to be measured, and determining frequencies f1, f2 and f3 from the FID signals; and second-order magnetic field variation compensating means for determining a second-order magnetic field variation xcex12 by solving the following simultaneous equations:
f1xe2x88x92f1r=xcex10+xcex11xc2x7r1+xcex12xc2x7r12
f2xe2x88x92f2r=xcex10+xcex11xc2x7r2+xcex12xc2x7r22
f3xe2x88x92f3r=xcex10+xcex11xc2x7r3+xcex12xc2x7r32
wherein the positions of said RF probes are designated by r1, r2 and r3, and adjusting the corrective magnetic fields based on said second-order magnetic field variation xcex12.
In the MRI apparatus of the fifteenth aspect, the second-order static magnetic field correcting method as described regarding the sixth aspect can be suitably implemented.
In accordance with its sixteenth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in comprising zeroth-order magnetic field variation compensating means for determining a zeroth-order magnetic field variation xcex10, and adjusting a transmit frequency for an RF pulse and a receive detection frequency for an NMR signal based on said zeroth-order magnetic field variation xcex10.
In the MRI apparatus of the sixteenth aspect, the second-order static magnetic field correcting method as described regarding the seventh aspect can be suitably implemented.
In accordance with its seventeenth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in comprising first-order magnetic field variation compensating means for determining a first-order magnetic field variation xcex11, and adjusting a gradient electric current based on said first-order magnetic field variation xcex11.
In the MRI apparatus of the seventeenth aspect, the second-order static magnetic field correcting method as described regarding the eighth aspect can be suitably implemented.
In accordance with its eighteenth aspect, the present invention provides the MRI apparatus of the aforementioned configuration, characterized in that said MRI apparatus is an open-type MRI apparatus that generates the static magnetic field in a vertical direction.
In the MRI apparatus of the eighteenth aspect, the second-order static magnetic field correcting method as described regarding the ninth aspect can be suitably implemented.
According to the second-order static magnetic field correcting method and the MRI apparatus of the present invention, second-order static magnetic field components can be reduced to improve homogeneity of the static magnetic field in an MRI apparatus.